Emulsion-aggregation (EA) toners are currently produced in a batch process in both laboratory and manufacturing scales. Batch processing can take many hours to complete. In addition, the reactors needed for batch processing may require a great amount of space. This results in a space-time yield, defined as the amount of particles produced/volume of reactor/time, that is relatively low when compared with most chemical processes. This is because the majority of the production time is spent heating the batch to the desired temperature set-points because of the reduction in heat transfer efficiency as scale increases.
The evolution of previous continuous EA reactor designs began with tubular, plug-flow reactors without any active agitation. These reactors were simple tubing in which a homogenized slurry was pumped through and heated. The resulting toner particles obtained showed very poor particle size distributions and were unable to produce a particle even remotely comparable to batch-produced toner. The nature of the aggregation mechanism in EA (orthokinetic flocculation) requires shear input in order to control particle size and prevent coarse formation. The lack of agitation in truly plug-flow reactors thereby inherently limits their performance without the addition of static mixers. Static mixers however, have the disadvantage of coupling the shear-rate, or mixing action, on the flow rate of the slurry; something that is undesirable from a process control standpoint.
However, efforts to carry out EA in a continuous process have shown space-time yields in an order of a magnitude higher than batch processing, but generally, as discussed above, at the expense of toner performance. Generally, the poor performance is due to poor toner particle size distribution. Poor particle size distribution is mainly due to the difficultly in mixing the viscous EA slurry during aggregation due to its high yield-stress. So called “dead-zones” created by poor mixing tend to broaden the particle size distribution and degrade the quality of toner produced.
In addition to toner performance, the complexity and scalability of the design of the reactor has also been an issue. Therefore, a need exists for a reactor design that produces a toner that is comparable a toner produced from batch processing, and is simple to operate, scale-up, fabricate, and maintain.
More current attempts at continuous aggregation have focused on utilizing a series of continuously stirred tank reactors. One such process is described in U.S. Patent Application Publication No. 2012/0183898, filed on Jan. 18, 2011, the entire disclosure of which is incorporated herein by reference. However, further improvement is still desired.
The method described herein overcomes the above described deficiencies, as well as many others, by providing a method for the aggregation of pre-toner particles utilizing homogenous mixing in a plug-flow manner. The method utilizes, for example, an Agitated Reactor Column (ARC) that is, for example, tubular in design yet provides homogenous mixing throughout its length by means of a perforated plate-type impeller. The ARC may be a single ARC, or may be two or more ARCs connected in series, for example, to aggregate both the core of the pre-toner particle and the shell independently. The method has the advantage of, for example, being scalable, easy to fabricate, and easy to operate.